The present invention relates to a method and apparatus for profiling surfaces and in particular vertical surfaces in a substrate using scanning force microscopy and specifically to a novel control mechanism for positioning a tip of a scanning force microscope for sensing and tracking the sidewall of a trench or line disposed in a substrate. Quite specifically, the instantaneous slope of the surface to be profiled is measured and used for controlling the scan track of the scanning force microscope tip.
The capability to measure line widths and to profile trenches in substrates is becoming more and more important in the field of micro-metrology. Presently, instruments based upon optical interaction are inaccurate or have physical limitations when the features to be measured are smaller than a micrometer.
Accurate micro-metrology is typically performed with scanning electron microscopes (SEM). There are several disadvantages to the use of a SEM, such as the need to perform measurements in a vacuum environment and cross-sectioning of the substrate where the measurements are performed at only one location of the groove or trench. SEM measurement is a time consuming process and there is limited spatial accuracy due to effects resulting from e-beam interaction with the material being measured.
The use of scanning tunneling microscopes to measure forces between a tip and a surface of an insulating material by simply mounting a scanning tunneling microscope tip on a cantilever beam is described in the article by G. Binnig et al entitled "Atomic Force Microscope", Physical Review Letters, Vol. 56, No. 9, pp. 930-933 (1986). The resultant instrument combines the principles of the scanning tunneling microscope and the stylus profilometer.
Subsequently, in an article by Y. Martin et al entitled "Atomic Force Microscope-Force Mapping and Profiling on a sub 100 .ANG. Scale", J. Appl. Phys., Vol. 61, No. 10, pp. 4723-4729 (1987) a modified atomic force microscope was described which enabled a precise measurement of the force between a tip and a sample over a tip sample distance in the range between approximately 30-50 angstroms. In a first application, the force signal was used to maintain constant tip-surface spacing for facilitating profiling with a spatial resolution of 50 angstroms. In a second application simultaneous measurement of force and surface profile was described.
A publication entitled "Microprobe-Based CD Measurement Tool", IBM Technical Disclosure Bulletin" Vol. 32, No. 7, p. 168 (December 1989) describes the use of an atomic force microscope designed for measuring trench depth and width. The system incorporates a two-dimensional laser heterodyne system which detects the change in resonance frequency of the vibrating probe tip as the tip approaches the surface. The probe tip may be vibrated in either the horizontal or vertical direction depending upon which surface is being approached. The probe tip is held stationary while the wafer or other part being measured is moved parallel or perpendicular to its surface and the displacement measured. The instrument design utilizes a three-point probe tip with well defined sensor points for detecting bottom, right and left edges of a trench. The tip is lowered into the trench and is then repeatedly moved left to right at a series of specific heights above the bottom of the trench. The trench width is thereby measured as a function of height and, thus, accurate measurements of trench dimensions can be made.
In the prior art techniques, the top edges of the trench are detected by first vertically moving the tip toward the top surface of the trench and then scanning the tip horizontally within the trench. The trench measurements are then performed by lowering the tip in the center of the trench. The tip is made to horizontally approach one of the sidewalls and then the other in order to measure the width at a specific depth location. The depth ofthe tip in the trench is changed and the two sidewall approach technique is repeated. At each depth location, two measurements are performed, as the tip approaches the first and then the second sidewall, which is time consuming. Moreover, a complex dual optical sensor is required for first performing the vertical and then the horizontal approaches.
In prior apparatus, the tip is vibrated in the horizontal direction as well as the conventional vertical direction, possibly at different frequencies, in order to sense the horizontal and vertical components of the force gradient. A serious limitation of such an arrangement is the need to measure the horizontal vibration. However, by using a second interferometer with its light beam focused on the side of the cantilever supporting the tip, the horizontal vibration be measured. Such an arrangement significantly increases the complexity of the apparatus.